1. Field of the Invention
This invention relates to an engine control system, and particularly to that for controlling injection of fuel.
2. Description of Related Art
Heretofore, in an engine of a type wherein a fuel is injected into an intake pipe, by installing an air-fuel sensor sensing the air-fuel ratio (A/F) of an exhaust gas upon combustion, the fuel injection quantity is subjected to feedback control in order to control the air-fuel ratio at a target air-fuel ratio. Accordingly, engine performance, exhaust gas characteristics, and fuel efficiency are improved. In the above fuel injection control system, when the A/F has changed from a fuel-lean state to a fuel-rich state, the fuel injection quantity is reduced, thereby gradually changing the A/F to a fuel-lean state. When the A/F has changed from a fuel-rich state to a fuel-lean state, the fuel injection quantity is increased. Accordingly, the A/F can be controlled at the target A/F.
In the above air-fuel ratio control system, it is possible to match the current air-fuel ratio to the target air-fuel ratio if the intake air quantity is accurately calculated and accordingly the fuel injection quantity is controlled. However, in practice, because the fuel injection quantity and the intake air quantity fluctuate for various reasons, a discrepancy between the current air-fuel ratio and the target air-fuel ratio occurs. All of the fuel injected into the intake pipe does not go into a combustion chamber, and a part of the fuel is deposited on a wall of the intake pipe. The fuel deposited on the wall is vaporized at a rate changeable depending on the time constant for evaporation regulated by engine revolutions and the temperature of the wall of the intake pipe. Further, the fuel deposition ratio indicating the quantity of fuel deposited on the wall changes in accordance with the engine operation. The air intake quantity also fluctuates with time depending on the engine conditions themselves such as the timing of valving and environmental changes around the engine including changes in intake temperature or ambient pressure (a change in air density).
In order to resolve the above problem, many sensors and control maps are required in the conventional feedback control to reduce the above-mentioned discrepancy in the A/F. Further, response characteristics suffer due to complex control flows. Thus, the conventional feedback control cannot achieve accurate air-fuel ratio control. Further, there is a dead time from the time fuel is injected to the time the fuel enters into the combustion chamber, and thus, during a transition state of the engine where the throttle angle changes widely, response characteristics suffer. Accurate air-fuel control cannot be performed.
An objective of the present invention is to provide a simple engine control system using a minimum number of sensors and performing accurate air-fuel ratio control, thereby solving the above problems and satisfying market requirements.